Add new options MARIADB_OPT_CONNECT_ATTR_*

The current implementation for connection attributes requires to call mysql_optionsv for each connection attributes instead of passing all connection attributes at once.

Since mysql_optionsv is a variadic function, the last passed parameter always needs to be NULL.

New options:

• MARIADB_OPT_CONNECT_ATTR_ADD adds one or more connection attributes
• MARIADB_OPT_CONNECT_ATTR_DEL deletes one or more connection attributes
• MARIADB_OPT_CONNECT_ATTR_CLEAR deletes all connection attributes

Example:

rc= mysql_optionsv(mysql, MARIADB_OPT_CONNECT_ATTR_ADD,

                   "_client_name", "libmariadb",

                   "_client_version", MARIADB_PACKAGE_VERSION,

                   "_os", MARIADB_SYSTEM_TYPE,

                   "_platform", MARIADB_MACHINE_TYPE,

                   NULL);